Radiation is one of the greatest challenges facing a mission to the red planet, planned by the United States and Europe in the first half of this century.

The shortest round trip would take at least 18 months, and during this time, the crew would be exposed to sub-atomic particles that whizz through space. These particles are capable of slicing through DNA and boosting the risk of cancer and other disorders.

The peril has been known for nearly half a century, but has seemed difficult to solve because costs and technological difficulty.

Some experts have toyed with the idea of shielding the crew with lead or massive tanks of water, but the price of lifting this load into orbit from earth is high.

Another idea would be to swathe the spaceship with a replica of earth's own magnetic field, deflecting incoming cosmic rays.

According to previous calculations, the spacecraft would have to generate a magnetic field hundreds of kilometres across.

But such equipment would be huge and drain the ship's energy supply and its powerful field could well harm the crew.

Bubble protection

British and Portuguese scientists have taken a fresh look at this old concept and say the magnetic field does not, in fact, have to be huge - just a 'bubble' a few hundred metres across would suffice.

"The idea is really like in Star Trek, when Scottie turns on a shield to protect the starship Enterprise from proton beams - it's almost identical really," says Bob Bingham of the Rutherford Appleton Laboratory in the UK.

Their research uses numerical simulations also used by experts in nuclear fusion, in which hot plasma is kept in place by a powerful magnetic field.

This technology provides an accurate picture of how individual particles behave when they collide with a two-pole magnetic field.

As a result, the researchers have been able to devise a smarter, miniaturised model of magnetic protection.

Scaled test

Using a plasma lab at the Superior Technical Institute in Lisbon, the team tested a scaled down version of the device in a simulation of a solar storm of atomic particles.

Scaled up for a trip to Mars, the device would weigh around "several hundred kilos" and use only about a kilowatt of energy, or around one half to one third of the typical power consumption of today's communications satellites, says Bingham.

The force of the magnetic field would replicate the earth's, but to minimise any risk to crew close to its source, could be carried in spacecraft flying either side of the crewed ship.

It would scatter almost all particles dispatched in "solar storms" - protons belched out by the sun, he says.

It would not work against a somewhat less dangerous problem, of high-energy cosmic rays that fly across interstellar distances, but the ship could be swathed with material, like a Kevlar bulletproof waistcoat, to protect against that threat.

"It certainly will be the answer if we go to Mars, because going to Mars will take about 18 months and we need to protect the astronauts against these storms," says Bingham.

In 2001, a NASA study found that at least 39 former astronauts suffered cataracts after flying in space, 36 of whom had taken part in missions beyond earth's orbit.

The agency has tentatively estimated that a trip to Mars and back would give a 40-year-old non-smoking person a 40% chance of developing fatal cancer after they returned to earth, or twice the terrestrial risk.